Let $A$ be an $5 \times 7$ matrix with rank 3 . If the the nullity of $A$ is $n$; the dimension of the row space of $A$ is $m$; the dimension of the column space of $A$ is $k$, then $n+m+k=$ ?
Using the rank-nullity theorem, we get $3 + n = 7$. Solving for $n$, we find $n = 4$. Thus, $n + m + k = 4 + 3 + 3 = \boxed{10}$.
Step 1 :Given a $5 \times 7$ matrix $A$ with rank 3, we have $m = k = 3$.
Step 2 :Using the rank-nullity theorem, we get $3 + n = 7$. Solving for $n$, we find $n = 4$. Thus, $n + m + k = 4 + 3 + 3 = \boxed{10}$.